NuPlayer: Allow audio callback to fill buffer when paused

If a pause lasts more than a few seconds, the audio device
enters standby.  When the audio device comes out of standby,
it typically requests a large amount of data to fill its buffers
(especially severe for deep buffer PCM mode).

To avoid a glitch, continue to service the audio callback
when paused.

Also, we do not start the audio sink if we are paused to avoid
audio playback in video preview mode.

Bug: 19062223
Bug: 21198655
Change-Id: I987337a87e6c875aeb381df8ad37c7139c42dc72